The land passed into English rule in 1664; the Dutch regained control in 1673, but the English took it back in 1674. Until 1776, the year of American independence, the county was under English or British control.[4] The county was not organized as a legal entity until after the Revolution in 1791, when it was created from an area that was originally part of the very large Albany County.

In 1807, in a county re-organization, the rural sections of Troy were set off as Towns, and the city was incorporated. The two towns created were Brunswick and Grafton, both named after British dukes, (the of Brunswick and Grafton). A third town, Philipstown, was set off in 1806. In 1808 it was renamed Nassau after the duke of that area.

There were 61,094 households out of which 33.30% had children under the age of 18 living with them, 46.80% were married couples living together, 12.00% had a female householder with no husband present, and 34.80% were non-families. 27.90% of all households were made up of individuals and 10.30% had someone living alone who was 65 years of age or older. The average household size was 2.46 and the average family size was 3.02.

In the county the population was spread out with 24.20% under the age of 18, 10.10% from 18 to 24, 29.10% from 25 to 44, 23.00% from 45 to 64, and 13.60% who were 65 years of age or older. The median age was 37 years. For every 100 females there were 95.90 males. For every 100 females age 18 and over, there were 93.70 males.

The median income for a household in the county was $42,905, and the median income for a family was $52,864. Males had a median income of $36,666 versus $28,153 for females. The per capita income for the county was $21,095. About 6.70% of families and 9.50% of the population were below the poverty line, including 11.90% of those under age 18 and 6.60% of those age 65 or over.

Rensselaer County Office building, which houses county offices, including that of the County Executive

Rensselaer County Courthouse located on the corner of Congress and 2nd Streets in Troy

Beginning in 1791, Rensselaer County was governed by a Board of Supervisors, which acted as the Legislature, with the chairman of the board serving as a de facto Executive.

In 1970, the Rensselaer County Legislature was created, which elected Edward J. "Ned" Quinn as Chairman. The Chairman served as the equivalent to an executive until the office of County Executive was created in 1972. Since its creation, Democrats have never won the office, although they controlled the Legislature until 1994. One notable candidate for Executive was Edward Pattison who was later elected to Congress, and whose son Mark served two terms as Mayor of Troy. The current county executive is Kathleen M. Jimino. Legislative authority is vested in the County Legislature, which consists of 20 members representing 17 different communities, separated into six districts. The current composition of the Legislature is as follows (11 Republicans, 6 Democrats and 3 Conservatives that caucus with the Republicans):

The county is serviced by 16 school districts. Some are completely contained in the county while some cross county lines into other counties. No school districts cross either the Vermont or Massachusetts state borders. Below is a table that shows the districts within the county, which BOCES they belong to, and which other counties they may serve.[12]